Strain gages for measuring strain in a test specimen or test material when the specimen is subjected to a mechanical force are well-known in the art. Strain produces relative displacement of spaced-apart points on the test specimen and can be measured. Resistive strain gages are commonly used, but are limited to measurement of relatively small strain ranges and are subject to creep and drift as they age.
Capacitive strain gages have also been utilized in the prior art. Typically, capacitive strain gages utilize capacitors with plates which are movable relative to each other as a function of applied strain. The plates of the capacitor are affixed by appropriate mounting means to different points on a test specimen. As force is applied to the test specimen, relative movement of the capacitor plates causes the capacitance to change. The change in capacitance is sensed by detecting the change in an applied a.c. signal. Capacitive strain gages have been utilized because of their capability to measure large strain ranges, stable operation over a wide range of temperatures and the lack of creep and drift with age. A prior art capacitive strain gage is disclosed in U.S. Pat. No. 4,197,753 issued Apr. 15, 1980 to Harting et al.
Various problems have been associated with capacitive strain gages having movable capacitor plates. A strain gage is intended to measure strain in a single direction. However, movable capacitor plates often move in directions other than the selected measurement direction due to mechanical imperfections, vibration and the like. Movement in directions other than the selected measurement direction causes a change in capacitance which results in an erroneous strain measurement. A further disadvantage of capacitive strain gages having movable capacitor plates is the necessity for connecting flexible lead wires to the movable plates. When the gage is subjected to vibration or to other mechanical stress, the flexible leads tend to break, causing a failure of the gage.
It is a general object of the present invention to provide improved capacitive strain gages.
It is a another object of the present invention to provide capacitive strain gages having capacitor plates with fixed relative positions.
It is a further object of the present invention to provide capacitive strain gages that are highly accurate over a wide range of operating conditions.
It is yet another object of the present invention to provide capacitive strain gages that are simple in construction and extremely rugged when subjected to adverse environmental conditions.